The ability to determine the sleep state of a patient, e.g., whether or not a patient is asleep, or whether a sleeping patient is within the rapid eye movement (REM), or one of the nonrapid eye movement (NREM) states (S1, S2, S3, S4), is useful in a variety of medical contexts. In some situations, the ability to determine the sleep state of a patient is used to diagnose conditions of the patient. For example, the amount of time that patients sleep or are within the various sleep states during sleep, the extent of arousals during sleep, and the times of day that patients sleep have been used to diagnose sleep apnea. Such sleep information could also be used to diagnose psychological disorders, such as depression and mania.
In other situations, a determination as to whether a patient is asleep is used to control delivery of therapy to the patient by a medical device, such as an implantable medical device (IMD). For example, neurostimulation or drug therapies can be suspended when the patient is asleep, or the intensity/dosage of the therapies can be reduced when a patient is asleep. As another example, the rate response settings of a cardiac pacemaker may be adjusted to less aggressive settings when the patient is asleep so that the patient's heart will not be paced at an inappropriately high rate during sleep. In these examples, therapy may be suspended or adjusted when the patient is asleep to avoid patient discomfort, or to reduce power consumption and/or conserve the contents of a fluid reservoir of an IMD when the therapy may be unneeded or ineffective. However, in other cases, a therapy intended to be delivered when the patient is asleep, such as therapy intended to prevent or treat sleep apnea, is delivered based on a determination that the patient is asleep.
In some cases, an ailment may affect the quality of a patient's sleep. For example, chronic pain may cause a patient to have difficulty falling asleep, disturb the patient's sleep, e.g., cause the patient to wake, and prevent the patient from achieving deeper sleep states, such as one or more of the NREM sleep states. Other ailments that may negatively affect patient sleep quality include movement disorders and congestive heart failure.
Further, in some cases, poor sleep quality may increase the symptoms experienced by a patient due to an ailment. For example, poor sleep quality has been linked to increased pain symptoms in chronic pain patients. The link between poor sleep quality and increased symptoms is not limited to ailments that negatively impact sleep quality, such as those listed above. Nonetheless, the condition of a patient with such an ailment may progressively worsen when symptoms disturb sleep quality, which in turn increases the frequency and/or intensity of symptoms. The increased symptoms may, in turn, further disturb sleep quality.
Because of the relationship between quality of sleep and symptoms, the quality of a patient's sleep may be indicative of the progression of an ailment and/or the effectiveness of a therapy delivered to treat the ailment. Assessing the quality of a patient's sleep in order to evaluate the progression of an ailment or the effectiveness of a therapy delivered to treat the ailment may involve identifying sleep states to, for example, identify disturbances in sleep, or difficulty in achieving or maintaining deeper sleep states. Consequently, it may be desirable for a medical device, such as an IMD, that treats the ailment of the patient to identify sleep states of patient.
The “gold standard” for identifying sleep states of a patient is polysomnography (PSG). PSG involves monitoring the electroencephalogram (EEG) of the patient over an extended period of time, e.g., overnight when the patient is attempting to sleep. Often, PSG also involves monitoring one or both of an electrooculogram (EOG) and a chin or jaw electromyogram (EMG) during the extended period of time. In some cases, PSG may additionally involve monitoring other physiological parameters of the patient, such as heart rate, respiration rate, and blood oxygen saturation level.
However, PSG is typically conducted in a clinical setting over the course of one night utilizing multiple sensors applied to a patient and coupled to a recording system. A technician or algorithm analyzes or “scores” the recorded sensor data to identify the sleep states of the subject during the night. Because EEG, EOG, and chin or jaw EMG monitoring typically require that an array of obtrusive external electrodes be placed on a patient's scalp and face and coupled to an external monitoring device, PSG is generally unsuitable for long-term monitoring of a patient's sleep states by a medical device, and particularly an IMD, as would be desired for identification of sleep states to control delivery of therapy or monitor the quality of a patient's sleep.